The present invention relates to a method for winning the valuable metal content of a leaching solution acidified with sulphuric acid, by solvent extraction and a following final product winning stage, for instance an electro-winning stage. The invention is particularly suitable for application in bioleaching processes, and is adapted thereto, although the invention is not limited to use in conjunction with such processes.
In respect of bioleaching processes for winning valuable metals, such as zinc, nickel and cobalt, from ores or concentrates and other materials that contain valuable metals for instance, it has been proposed in the literature that metal is won from the leaching solution deriving from a bioleaching process, which is an acid leaching process in the presence of bacteria, with the aid of solvent extraction and electrowinning. Bioleaching is described more generally in our earlier patent publications U.S. Pat. No. 5,397,380 and SE-A-9901613-1 and also in WO 9216667 for instance, which deals generally with the oxidation of metal sulphide material with bacteria, and in WO 94/28184 which relates to the bioleaching of zinc concentrate.
An article written by A. P. Briggs et al (Int. Biohydrometallurgy Symposium IBS97, Sydney 1997) describes a bacteria leaching process in Uganda for winning cobalt from roasted pyrites, where the cobalt content is won by solvent extraction followed by electrowinning (SX-EW).""
Another article written by M. L. Steemson et al and included in the same publication (IBS97) describes a bioleaching process for treatment of zinc concentrate, where the zinc is won by solvent extraction followed by electrowinning (SX-EW). A similar process is also described in AU-B-673927.
A common feature of these earlier proposed methods which include a solvent extraction electrowinning stage (SX-EW) is that solvent extraction is performed on the leaching solution subsequent to precipitating out the iron present therein. One drawback with these proposed methods is that a major part of the leaching solution taken from the solvent extraction stage and depleted of valuable metals, this solution normally being referred to as a raffinate, must be returned to the bioleaching stage in order to avoid the release of metals to the recipient, and therewith also reduce unnecessary valuable metal losses. Alternatively, the valuable metal concentration of the so-called raffinate must be very low, namely a low content that is made possible to obtain solely by part-processes and thus with multi-stage extraction. The drawbacks of such a process will be clearly apparent, both from the aspect of apparatus and from the aspect of the work required. In addition, recycling of the raffinate to the various part-process stages is complicated process-wise and the effect of the process is difficult to oversee and control. A significant question is what happens to the activity of the bacteria cultures in the bioleaching circuits. There can occur in the reactors in the bioleaching stage an acid surplus that requires the addition of large quantities of lime or limestone to neutralise the acid, this addition causing, in turn, problems with respect to the viscosity of the pulp in the circuits due to the resultant precipitation of gypsum. So-called xe2x80x9csurplus waterxe2x80x9d must also be removed from the solvent extraction stage together with the raffinate, and because it has been in contact with the organic extraction agent utilised as a binding medium between the incoming liquid phase rich in metals and the incoming metal-lean liquid phase, it will contain relatively significant quantities of said binding medium, for instance. Normally, this amount is about 100 ppm organic phase in the raffinate in the case of heap leaching.
It has now been found possible to provide a method for winning the valuable metal content of a leaching solution acidified with sulphuric acid, by solvent extraction followed by a final product winning stage, for instance an electro-winning stage, with which the aforedescribed drawbacks are eliminated at least substantially, and with which a more readily controlled and flexible process is obtained. The inventive method comprises the steps, or stages, set forth in the accompanying Claims.